1. Field of the Invention
The present invention relates to a developing apparatus that develops an electrostatic latent image formed on an electrostatic latent image carrying body using a nonmagnetic toner in electrophotographic copying machines, printers, and the like.
2. Description of the Related Art
U.S. Pat. No. 2,297,691 discloses an electrophotographic method of forming an electrostatic latent image by exposing an image to a photosensitive body having a photoconductive insulating layer that is electrically charged uniformly and then rendering such electrostatic latent image into a visible image by causing, e.g., fine powdered electrically sensitive particles called toner to adhere thereto.
U.S. Pat. No. 2,874,063 discloses a typical magnetic brush development method that uses a two-component developer comprising a toner and a carrier which is made of magnetic particles such as brown ion oxide, ferrite or the like. The toner in the developer is electrically charged by friction at the time the carrier and the toner are being stirred and the like. The electrically charged toner is then held by magnetic attraction of a magnet contained in a developing roll so that the toner particles are oriented so as to be brushlike in the presence of a magnetic field of the magnet at a position where the toner particles confront the photosensitive body. Such magnetic brush comes in contact with an electrostatic latent image formed on the photosensitive body, and the toner in the developer adheres to the photosensitive body to develop an image.
The above-mentioned magnetic brush development can impart sufficient electric charge to the toner in the developer, and the toner can be supplied to the developing roll stably. Therefore, this development method is successful in producing uniform and satisfactory image quality at the initial state of copying. However, in the case of development for several tens of thousands of copies (e.g., copying operations), problems such as reduction in image density, fog in background areas and the like are addressed. The reason therefor is ascribed to the fact that the developer is subjected not only to magnetic attraction of the developing roll but also to compression or shearing due to collision during stirring or to reduction in the capability of the carrier to electrically charge the toner due to the surface of the carrier being contaminated either by the toner or an externally added agent used as an electric charge control agent or a fluidity imparting agent.
Further, the magnetic brush development method that causes the magnetic brush to contact the electrostatic latent image on the photosensitive body as described above produces sweep pattern by the rubbing of the toner image first developed on the photosensitive body when applied to a full color copying machine, thereby imposing the problem of not implementing high-quality color copying.
On the other hand, U.S. Pat. No. 2,895,847 discloses another electrophotographic development method as a touchdown development method which is designed to hold a nonmagnetic toner on a toner carrying body called donor and effects development with the toner contacting or not contacting an electrostatic latent image. As a touchdown development method, U.S. Pat. No. 3,866,574 discloses a technique characterized as generating an AC electric field in a development area for development by noncontact with the photosensitive body. Unexamined Japanese Patent Publication No. Sho. 54-51848 discloses a method that a layer forming member comprising a soft elastic body is compressedly contacted on the surface of the donor.
Since the touchdown development methods allow development to be effected with the toner on the donor in thin layer form adhering to the photosensitive body in a noncontact manner, it has an advantages to be possible to produce high-quality color copies. However, the touchdown development methods still have the following problems.
In the touchdown development methods without the magnetic carrier as in the magnetic brush method, the toner cannot ensure supply, to the donor causing erratic image density. Further, because only a triboelectricity at a portion where the donor contacts compressedly with the layer forming member is for charging the toner, a small amount of electric charge is stored, whereby a so-called "high .gamma. image" in which the reproduced image density is lower than that of the original or the toner splashes to dirty the machine body. Still further, in forming a layer of the toner on the donor, the toner is subjected to stress and frictional heat as the toner passes through the area in which the donor comes in pressure contact with the layer forming member. As a result, an externally added agent such as an electric charge control agent or a fluidity imparting agent is hidden in the toner with the function thereof lost, thereby bringing about reduction in image density, fog in background areas, and the like.
To overcome the above-mentioned problems, U.S. Pat. No. 3,929,098 discloses a method for supplying a nonmagnetic toner onto the donor surface by the magnetic brush development method with a magnetic carrier. That is, as shown in FIG. 1, this development method involves the steps of: forwarding a developer composed of a nonmagnetic toner a and a magnetic carrier b with the developer being held on a developer forward member d having a built-in magnet roll c therein in the shape of a magnetic brush by magnetic attraction to a position where the developer confronts a donor (toner carrying body) e; delivering only the nonmagnetic toner a in the developer onto the donor e to form a layer by an electric field generated by a voltage applied from a power supply f across the developer forward member d and the donor e; and developing an electrostatic latent image of an electrostatic latent image holding body g by such toner a thereafter. In FIG. 1, reference characters h and i designate a developer forward stirring member and a frame of a developing apparatus, respectively.
According to this method, not only the nonmagnetic toner can be supplied to the donor stably, but also a uniformly thin layer can be formed by appropriately controlling the electric field for delivering the toner to the donor. However, there still remains unsolved problems that the apparatus becomes complicated and thus costly, and that the contamination of the carrier peculiar to the magnetic brush development method leads to a reduction in the toner charging amount and hence to a reduction in image density as well as fog in background areas.